GCS notification convenience methods, pubsub dependency

[tseaver]

In #4021, @BrandonY asked for convenience wrapprer(s) for the following:

• Create the user-supplied topic
• Set the appropriate IAM policy needed to support bucket notifications
• Create a subscription to the topic

Adding this support would make storage depend on pubsub (at least optionally), and would complicate our testing story (system tests already add it). Ruby just documents what is required.

@danoscarmike, @lukesneeringer nominated you as the arbitrator for the issue.

[BrandonY]

Without this convenience function, everyone who wants to use notifications will have to write a function that invokes around 4 calls:

• Call GCS to request the name of the appropriate permission for your GCS bucket.
• Call Pub/Sub to create a topic
• Call Pub/Sub's IAM interface to add publish permission to the robot that was the response to the first RPC.
• Call GCS to add notifications to a bucket.

With a convenience method, the whole thing can be shortened to bucket.sendNotifications(topic).

[danoscarmike]

Ack. @BrandonY I'll reach out to you offline.

[lukesneeringer]

This was decided. We are not doing it.